Pricing is $13/month for additional TiVos on your account. You cannot buy the lifetime service from Best Buy, just the TiVos themselves. You should be able to buy one more TiVo and get the $399 lifetime. If you buy another TiVo to replace the S3, that one will be $13.99/month, not $6.95.

You can buy a Premiere at Best Buy and when you go to activate it on tivo.com, they will offer you $400 lifetime.

However, your $6.95 Premiere is tied to your S3 box. So, if you sell your S3 box, you no longer have a full priced unit to qualify for the $6.95 rate and your original Premiere will revert to a higher monthly price (i.e. $15, but maybe they'd give you the current MSD price of $13).

If you opt to pay full price ($500) for lifetime on the new Premiere, then maybe they'd let you keep the $6.95 rate on your original Premiere, if you sell your S3.